1. Field of the Invention
The present invention relates to stabilized power supplies called chopped power supplies.
A chopped power supply operates in the following way: a transformer primary winding receives a current which comes for example from a rectifier bridge receiving the power from the AC mains. The current in the transformer is chopped by a switch (for example a power transistor) placed in series with the primary winding.
A circuit controlling the transistor establishes periodic square waves for enabling the transistor. For the duration of the square wave the current is allowed to pass; outside the square wave, the passage of the current is prevented.
On one (or more) secondary windings of the transformer an AC voltage is then collected. This voltage is rectified and filtered so as to obtain a DC voltage which is the DC output voltage of the chopped power supply.
To stabilize the value of this DC voltage, the cyclic periodic conduction ratio of the switch is adjusted, that is to say the ratio between the conduction time and the disablement time in a chopping period.
2. Discussion of Background
In a chopped power supply architecture proposed by the applicant and shown in FIG. 1, two integrated circuits are used. One of the circuits, CI1, serves for controlling the base of a power transistor T.sub.p for applying thereto periodic enabling and disabling control signals. The space control circuit CI1 is placed on the primary winding side (EP) of the transformer (TA) for reasons which will be better understood further on in the description. The integrated circuit, regulation circuit CI2, is on the contary placed on the secondary side (winding ES1) and its serves for examining the output voltage Vs of the power supply for elaborating regulation signals which it transmits to the first integrated circuit through a small transformer TX. The first integrated circuit CI1 uses these regulation signals for modifying the cyclic conduction ratio of the switching transistor TP and thus for regulating the output voltage Vs of the power supply.
We will come back in more detail hereafter to the circuit shown in FIG. 1.
Numerous problems arise during designing of a chopped power supply, and the problems with which we will be particulary concerned here are problems of starting up the power supply and problems of safety should over voltages or over currents occur at different points in the circuit. The first problem which is met with is that of starting up the power supply : on switching on, the regulation circuit CI2 will tend to cause the base control circuit CI1 to generate square waves of maximum cyclic ratio until the power supply has reached its nominal output voltage. This is all the more harmful since there is then a heavy current drain on the side of the secondary windings which are connected to initially discharged filtering capacitors. There is a risk of destruction of the power transistor through over-currents during the start-up phase.
Progressive start-up circuits have already been proposed which limit the duration of the enabling square waves during a start-up phase, on switching on the device; the U.S. Pat. No. 3,959,714 describes such a circuit in which charging of a capacitor from switch-on defines initially short square waves which gradually increase in duration until these square waves reach the duration which the regulation circuit normally assigns thereto. The short square waves have priority; but, since they become gradually longer during the start-up phase, after a certain time they cease to have priority; this time is defined by the charging time constant of the capacitor.
Another problem which arises is the risk of accidental overcurrents, or sometimes overvoltages which may occur in the circuit. These over-currents and over-voltages may cause damage and often result in the destruction of the power transistor if nothing is done to eliminate them. In particular, a short circuit at the output of the stabilized power supply rapidly destroys the power transistor. If the short circuit occurs on start-up of the power supply, it is not the gradual start-up system with short square waves which gradually increase which will allow the over-currents resulting from this short circuit to be efficiently accomodated.
Finally, another problem, particularly important in an architecture such as the one shown in FIG. 1, is the risk of disappearance of the regulation signals which should be emitted by the regulation circuit CI2 and received by the base control circuit CI1: these signals determine not only the width of the square waves for enabling the power transistor but also their periodicity; in other words, they serve for establishing the chopping frequency, possibly synchronized from a signal produced on the secondary side of the transformer. The disappearance of these signals causes a particular disturbance which must be taken into account.
Furthermore, the architecture of FIG. 1, in which the secondary circuits have been voluntarily separated galvanically from the primary circuits, is such that the base control circuit may function rapidly after switch on, as will be explained further on, whereas the regulation circuit CI2 can only function if the chopped power supply is in operation; consequently, at the beginning, the base control circuit CI1 does not receive any regulation signals and this difficulty must be taken into account.